As a school governor myself I can understand where you are coming from however, unless you son's school is a private school, it is likely that your Internet connection is provided by your local authority (Ours is) and therefore is a cost that you have no control over. I too was shocked by the cost of some of the IT equipment that is installed. Our school is a small village primary with ~100 pupils and when we needed a new network as part of a building project, the original spec was £120k+ of very nice stacked Cisco switches, all gigE PoE capable. Nice but complete over kill for the school and I consider to be a waste of money (We don't have anything with even a single gig interface!). I appreciate that they put things in for future proofing but we could have had some nice HP units (For example) for a fraction of the price and replace them every 5 years to keep up with technology for less money but I digress...
If you really have the flexibility to source this from where ever, then there are options but again your local authority's Children's Services will no doubt have a set of criteria that needs to be complied with, for example, speed of circuit. If the school is a primary school, you should find you can get a away with DSL (Or bonded DSL for the bandwidth/ reliability) but secondary schools will almost always be on leased lines as they need the larger bandwidth.
Looking at the Becta requirements, although we don't hold their accreditation, I believe we can meet their criteria by using a combination of our usual connectivity solutions (DSL, bonded DSL & leased line) with our firewalls of choice, the Sonicwall range, which have a range of subscription based content and malware filters (See www.sonicwall.com/uk/5145_3059.html
for details). We can do this as a managed solution, meaning that we become responsible for delivering clean Internet out of a hole in the front of a box and the school doesn't need to know how.
If this is of interest and you think you can 'do it your self', I would be happy to arrange trials. On the other hand, if you would like to drop me a line, we can just compare school Internet connection costs